Methacrylic acid and methacrylic esters such as methyl methacrylate and butyl methacrylate are likewise used in a wide variety of applications. Typical end applications involve acrylic polymer webs or films, molding resins, polyvinyl chloride modifiers, processing aids, acrylic coating materials, floor care compositions, sealants, automotive transmission fluids, crank case oil modifiers, motor vehicle coatings, ion exchange resins, cement or adhesive modifiers, water treatment polymers, electronic adhesives, metal coatings and acrylic fibers.
The known industrial scale processes for preparing (meth)acrolein and/or (meth)acrylic acid (for example according to DE-A-19 62 431) by gas phase oxidation of propene or isobutene are generally performed in tube bundle reactors which have a large number (in some cases more than 30 000) reaction tubes welded in between tube plates. Preference is given here to working at temperatures between 200 and 450° C. and optionally elevated pressure. The reaction tubes are filled with the heterogeneous oxidation catalyst in the form of a fixed bed and the reaction mixture flows through them. The fixed beds preferably comprise catalyst materials which are based on mixed metal oxides and have been shaped to spheres, rings or cylinders, or else coated catalysts which have been obtained by the coating of preshaped inert support bodies with a catalytically active material.
The objective of any two-stage fixed bed gas phase oxidation of propene to acrylic acid or of isobutene to methacrylic acid, or else of a conversion of propane or isobutane to acrylic acid or methacrylic acid via a dehydrogenation followed by a two-stage fixed bed gas phase oxidation or else via a direct oxidation of the propane or isobutane, is in principle to achieve a maximum space-time yield of acrylic acid or methacrylic acid (STYAA or STYMAA) (this is the total amount of acrylic acid or of methacrylic acid obtained per hour and total volume of the catalyst bed used in liters in a continuous procedure). There is therefore a general interest in performing such a two-stage fixed bed gas phase oxidation of propene to acrylic acid or of isobutene to methacrylic acid firstly with a maximum space velocity of propene or isobutene on the first fixed catalyst bed (this is understood to mean the amount of propene or isobutene in standard liters (=1 (STP); the volume in liters that the corresponding amount of propene or isobutene would occupy at 25° C. and 1 bar) which is conducted as a constituent of the starting reaction gas mixture per hour through one liter of catalyst bed), and secondly with a maximum space velocity of acrolein or methacrolein on the second fixed catalyst bed (this is understood to mean the amount of acrolein or methacrolein in standard liters (=1 (STP), the volume in liters that the corresponding amount of acrolein or methacrolein would occupy at 25° C. and 1 bar) which is conducted as a constituent of the starting reaction gas mixture per hour through one liter of catalyst bed), without significantly impairing the conversion of propene or isobutene and acrolein or methacrolein which proceeds in the course of single pass of the starting reaction gas mixture through two fixed catalyst beds, and the selectivity of the associated acrylic acid or methacrylic acid formation (based on propene or isobutene converted) assessed over both reaction stages.
The achievement of the above is impaired by the fact that both the fixed bed gas phase oxidation of propene or isobutene to acrolein or methacrolein and the fixed bed gas phase oxidation of acrolein or methacrolein to acrylic acid or methacrylic acid first proceeds strongly exothermically and is secondly accompanied by a variety of possible parallel and subsequent reactions. With increasing propene or isobutene velocity and with increasing acrolein or methacrolein velocity on the particular fixed catalyst bed, it therefore has to be assumed, when the desired boundary condition of an essentially uniform propene or isobutene conversion and of an essentially uniform acrolein or methacrolein conversion is achieved, that, owing to the increased production of heat, the selectivity of product of value formation decreases (see, for example, also EP-A-450 596).
In order to improve the selectivity of product of value formation, it is therefore necessary to effectively remove the heat of reaction released in the conversion of the propene or of the isobutene and that released in the conversion of the acrolein or methacrolein. In the tube bundle reactors described at the outset, at least one heat exchange medium is therefore conducted around the reaction tubes on the shell side of the tube bundle reactor, which is, for example, a salt melt.
However, the above-described prior art tube bundle reactors have numerous disadvantages.
Firstly, in spite of the use of heat exchange media, the heat of reaction released can only be removed to a limited degree in the fixed catalyst beds, which is attributable especially to the poor heat removal of the fixed beds themselves. This poor heat removal of the fixed beds currently limits the tube diameter to approx. 2.5 to 3 cm, which necessitates the use of in some cases more than 30 000 reaction tubes in order to ensure a satisfactory space-time yield. However, owing to the large number of welded joints, this high number of reaction tubes causes high capital costs. This high number of reaction tubes also causes very complex catalyst filling procedures, in which it has to be ensured that the same amount of catalyst and a comparable bulk density is introduced in each of the reaction tubes. This is important since the pressure drop is otherwise not the same in all reaction tubes. A homogeneous pressure drop in the reaction tubes is one factor which is crucial for them ensuring a homogeneous reactant conversion and the minimization of by-product formation.
Moreover, in spite of the use of the heat removal medium, even in the case of tube diameters of only 2.5 to 3 cm, a temperature profile of in some cases more than 90° C. still develops in axial direction of the reactor. This results in so-called “hotspots” (these are regions with particularly high evolution of heat) in the reaction tubes, which more particularly also limits the lifetime of the catalysts in these regions. The difference between the temperature of the heat exchange medium and of the catalyst in the hotspot increases with increasing throughput. The formation of these hotspot temperatures can also lead to a decrease in the selectivity in the product conversion. To improve the selectivity with which the target products are formed, the prior art therefore proposes performing the heterogeneously catalyzed partial gas phase oxidation of acrolein to acrylic acid or of methacrolein to methacrylic acid, the dehydrogenation of propane or isobutane to propene or isobutene, the oxidation of propene or isobutene to acrolein or methacrolein, or else the direct oxidation of propane or isobutane to acrylic acid or methacrylic acid, as a multizone method (e.g. two-zone method) in a multizone tube bundle reactor (e.g. in a two-zone tube bundle reactor). In this case, a plurality of (e.g. two) essentially spatially separate liquid heat exchange media (which are normally of the same type) are conducted within the space surrounding the reaction tubes (these may, for example, be separated by separating tube plates which are inserted within the space surrounding the reaction tubes and have corresponding passage orifices for the reaction tubes).
A further disadvantage of conventional tube bundle reactors is that variations in the temperature of the heat exchange medium have a very strong and direct effect on the temperature in the reaction region of the reaction tubes, which can easily lead to runaway of the reactors. Conventional tube bundle reactors also have the disadvantage that the main reaction takes place in the above-described hotspot regions, whereas the temperature for a reaction is usually too low in the rear part of the reactor (this makes up about ⅔ of the total length of the reactor). Therefore, a large amount of catalyst material and a long residence time are needed in the rear part of the reactor to achieve a sufficient conversion.
Finally, the fixed catalyst beds used in the tube bundle reactors known from the prior art frequently lead to a high pressure drop, which necessitates an adjustment of the compressor output.
It was an object of the present invention to overcome the disadvantages arising from the prior art in connection with the direct oxidation of saturated hydrocarbons, and disadvantages which arise with the catalytic dehydrogenation and subsequent catalytic gas phase oxidation of saturated hydrocarbons, and the catalytic gas phase oxidation of unsaturated hydrocarbons or unsaturated aldehydes.